Method for producing an electrical component

ABSTRACT

To expose a submerged bondable terminal pad in a component that includes at least two substrates which are joined with each other, it is proposed that grooves of relatively shallow depth be provided on the connecting surface of the second substrate before the two substrates are joined. After the two substrates are joined, incisions are made opposite the grooves which open the grooves from the back side. The cutouts to be removed are defined between two grooves.

The present invention relates to a method for producing submergedbondable terminal pads on a component which includes at least twointerconnected substrates.

It is known from published patent specification WO98/05935A1 thatsensors can be placed on a substrate in such a way that the sensorelements are positioned over an opening in the substrate. It is knownfrom published patent specification JP 2000195827A as well as in thequotation in Patent Abstracts of Japan to place grooves on one side of asubstrate, and in order to separate the LED components formed on a waferto make incisions from the back side of the substrate over the groovesto such a depth that the grooves are opened. In the case of twoconnected substrates, it is known from published patent specificationJP09223678A that the procedure is to remove cutouts from a secondsubstrate to separate the components.

Examples of components that are realized in two connected substrates areso-called COC (chip on chip) components which have a type of verticalintegration. The two substrates which both component structures can haveare interconnected through two main surfaces, while at the same time, ifnecessary, an electrical contact can be established between componentstructures in different substrates. The component can have common orseparate electrical contacts for both connected substrates. If there areelectrical contacts on both substrates, the mutual electrical connectionof the component structures realized in the two substrates can also bemade by means of bonding wires between the respective contacts.

The COC technology is employed in particular for component encapsulationat the chip level, also known as “first level packaging”. It is alsoused where further miniaturization of the components is desired, and inparticular where components with less need for space on a circuit boardor in a module are sought. The technology is used in particular toproduce ICs, micromechanical components or sensors (MEMS), as well asfor producing micro-optical components.

The substrates of the COC components are usually produced by means ofintegrated methods at the wafer level, and the two component substratesare also connected at the wafer level. It is therefore necessary, on thelower of the two substrates, or in the case of a larger number ofsubstrates, on a lower substrate, to expose the electrical terminal padsafter the two substrates have been connected, and to that end forexample to produce a window in the top substrate. If the window isproduced after the two substrates are connected, there is a danger thatthe process of opening the window to expose the contacts (terminal pads)will cause the latter or other structures on the lower of the twosubstrates to be damaged or even destroyed.

It is therefore known and usual to form the windows in which theterminal pads are exposed in the upper one of the two wafers before thesubstrates are connected. In the case of semiconductor wafers, that canbe done for example by anisotropic etching from one or two sides of thetop wafer. As the etching technique, it is possible for example toemploy reactive ion etching. However, a disadvantage of the method isthat the top substrate is weakened as a result of the pre-producedwindow, possible breaking points are already predefined as with thewindows. This is particularly disadvantageous for the joining of the twowafers, which takes place in particular under pressure. The result ofthis is that COC technology, in which two substrates are joined togetherby bonding, is limited at the wafer level to wafers with a diameter offour to a maximum of six inches.

An additional disadvantage of this method is that precisely locatedetch-through of the windows requires a great expenditure of time andprocessing.

The object of the present invention is therefore to specify a method, inparticular for COC technology, which enables simpler exposure ofsubmerged terminal pads on components that include at least twoconnected substrates.

This problem is solved according to the present invention by a methodhaving the features of claim 1. Advantageous embodiments of theinvention may be seen from additional claims.

In contrast to the known method, in which the windows in the topsubstrate are produced in a complicated step by etching before the twosubstrates are joined, the exposure of the terminal pads is performedaccording to the present invention in two steps.

A first step, which is performed before the substrates are joined,consists of producing grooves in the surface of the second (upper)substrate. These grooves define and delimit a cutout in which theterminal pads of the first or lower substrate are later exposed. Thegrooves are produced to a specified depth, which corresponds to only afraction of the total thickness of the substrate.

The first substrate, which has on its surface the forenamed terminal padfor component structures located in the first substrate, is then joinedwith the second (upper) substrate in such a way that the surface of thesecond substrate with the grooves faces the surfaces of the firstsubstrate that has the terminal pads.

Next, incisions are made from the back side of the second uppersubstrate in the area over the grooves; these incisions extend to adepth such that in the incisions the grooves are opened along theirentire length. Preferably, the incisions are also made parallel to thegrooves, or directly over the grooves and following the course of thegrooves.

The method according to the present invention has the advantage thatbecause of the only slight depth of the grooves the top wafer retainssufficient strength, significantly lowering the risk of a substrate orwafer break compared to the known method. The method has the additionaladvantage that the more complicated sub-step of the method, namely theproduction of the grooves, can be performed significantly more quicklyand with less complication because of their shallow depth. Producing theincisions from the back side on the other hand can now be done using amore cost-effective, simpler and possibly more imprecise method, forexample by sawing.

After the grooves have been opened in the incisions, the secondsubstrate is severed in the area of the cutout, so that the cutout canbe removed, which produces the desired window in which the terminal padsare exposed.

If a sawing procedure is used to produce the incisions, and if forexample a diamond saw is used, it is advantageous to produce theincisions as straight-line cuts. Correspondingly, straight-line groovesare also provided in the opposing surface of the second substrate.Matching this, preferably the terminal pads are arranged side-by-side ina row, so that with a single cutout or with two incisions an elongatedwindow that extends over the entire substrate can be opened, in whichone or more rows of terminal pads can be exposed in a single step.

A row of terminal pads can belong to one or more components positionedside-by-side on the first substrate.

The grooves can be produced using any desired procedure which can becarried out with structure and control in such a way that preciselypositioned grooves of a desired groove depth can be produced. It ispossible for example to define the grooves by means of a resistance maskwhich is photolithographically structured. With the resistance mask, astructured etching process is then carried out, in which material isremoved in the areas not covered by the resistance mask until a desiredor prescribed groove depth has been reached. For the etching process, itis possible to use wet chemical etching, ion beam etching orplasma-enhanced etching. The etching can be done isotropically oranisotropically, since the groove profile and groove width have almostno influence on the quality of the component produced with the methodaccording to the present invention. Anisotropic methods are preferred,however, since less material needs to be removed in such methods, whichreduces the expense of the etching process.

The reason that the width of the grooves has no influence on the resultof the process is that the grooves serve merely to define the cutout,and because all that has to happen in the grooves is a separation of thesubstrate to the depth of the grooves. A groove width is advantageouswhich produces reliable separation to the desired depth, so that afterthe incisions have been made no bridges of material remain between thecutout and the remaining substrate.

Advantageously, measures are provided that guarantee easy detachment orcomplete removal of the cutout, that is, of the area of the secondsubstrate that is defined between the grooves and that forms the windowfor exposing the terminal pads. One possibility for this is to provide afree space over the terminal pads, which also ensures a clear intervalbetween the terminal pads and the adjacent surface of the second waferafter the substrates have been joined.

To ensure such a clear interval, in at least one of the two substrates astructure can be produced on one of the two opposed surfaces whichserves as a spacer beyond the window. It is also possible to provide acorresponding depression under the terminal pads in the first substrateor over the terminal pads in the second substrate.

An additional possibility for simplifying the complete removal of thecutout is to provide a cover over the terminal pads in the area of thecutout, which prevents direct adherence of the second substrate to theterminal pads when the two substrates are joined. Preferably, one usesfor this purpose covers that can be removed easily after the cutout hasbeen opened, and which therefore should exhibit little or no adhering tothe two substrates. Especially suitable as covers therefore are coatingsor coating materials that develop only weak adhesion forces on thesubstrate, or which produce a layer with only weak internal cohesion, sothat it is possible to remove the cutout easily.

In principle it is also possible to employ methods for joining the twosubstrates which result intentionally and selectively in the productionof a firm connection between the two substrates only at desiredconnecting surfaces. These include in particular procedures in which aconnecting material is applied to one of the two substrates to producethe connection. Examples of these are glass bonding, bonding by means ofbumps, eutectic bonding and to a certain degree also anodic bonding.With these connecting procedures it is not necessary to ensure a freeclearance of the two substrates over the terminal pads or to provide acover layer over the terminal pads. These methods can be carried out insuch a way that the connecting positions of the two substrates occuroutside of the surfaces of the substrates intended for the terminalpads. Also suitable for joining the two substrates is a glue, and if thesubstrates include semiconductor material, direct bonding of thesesemiconductor materials. To that end, they are aged at a hightemperature, which produces a firm connection of the semiconductorsurfaces that come into contact with each other.

The invention will now be explained in greater detail on the basis ofexemplary embodiments and the matching figures. The figures representonly schematic portrayals; they are not executed true to scale, and canreproduce only parts of the inventions on the basis of the specificexemplary embodiments.

FIG. 1 shows a schematic cross section of a known COC component,

FIG. 2 shows a schematic cross section of two substrates which are to bejoined,

FIG. 3 shows the arrangement of the grooves on the second substrate,

FIG. 4 shows a schematic cross section of the components duringproduction of the incisions,

FIG. 5 shows the component after the incisions,

FIG. 6 shows a top view of the terminal pads exposed on the firstsubstrate,

FIG. 7 shows an individual component after separation.

FIG. 1 portrays a schematic cross section of a known chip-on-chipcomponent. The component is made up of a first substrate S1 and a secondsubstrate S2 connected to it, which are joined with known bondingtechnologies. At least the lower first substrate S1 contains componentstructures which are electrically connected with the outside world viathe terminal pad AF on the top of the lower substrate S1. In order forthe latter to be accessible from the outside in the two-layer component,the second substrate S2 has a window in the area of the terminal pad AF,in which the terminal pad AF is exposed. The window in the secondsubstrate S2 is produced by means of etching technologies before the twosubstrates are joined.

FIG. 2: According to the present invention, the following procedure isused in a component made up of at least a first substrate S1 and asecond substrate S2. Grooves G are produced in the second substrate S2with an etching technique. Between two straight-line grooves or within acircular closed groove structure, a cutout AS is defined whichcorresponds to the size of the window which will be opened later. On thesurface of the first substrate S1 there are terminal pads AF, forexample solderable contacts. At least one of the two substrates has onone of the surfaces to be joined a spacing structure A, which in thepresent case for example represents the connecting point between the twosubstrates S1 and S2. The spacing structure A can be for example a bump,a metallizing for eutectic bonding, or an application of glue. Thespacing structure A can be made by structuring the surface of one of thesubstrates. To that end one of the substrate surfaces is structured byetching in such a way that a suitably higher level is produced at thepoints intended for bonding, compared to the rest of the surface of thesubstrate.

FIG. 3 shows the substrate S2 in a top view of the surface in which thegrooves G are arranged. In the substrate, in which a plurality ofindividual components are realized, the boundaries between the futureindividual components are identified by dashed separating lines TL_(h)and TL_(v). The grooves G preferably run in straight lines over theentire substrate and are arranged in pairs in such a way that theydefine a strip-shaped cutout AS between them. The strip-shaped cutout ASis preferably positioned directly over a separating line TL_(v), so thatthe cutout AS covers the edge areas of two adjacent individualcomponents. With the structuring shown in FIG. 3, only one strip-shapedcutout AS is produced in each individual component. It is also possible,however, to produce two or more of the cutouts per individual component,if a larger number of terminal pads need to be exposed in them.

After the grooves G have been produced, the two substrates S1 and S2 arejoined with each other according to a known method, in particular awafer bonding method or by gluing. In the example shown in FIG. 4, theconnection is made through the structures A. It is also possible howeverto join the two substrates over a large area, and to that end inparticular to position the terminal pads AF in an indentation. It isalso possible to provide an indentation in the second substrate S2 overthe terminal pads AF.

With the help of a saw, for example a diamond saw DS, which isrepresented in the figure only schematically as a rotating disk, anincision ES is now produced over the grooves G from the back side of thesecond substrate S2 opposite the grooves. The incision ES is made to adepth such that the particular groove is opened from the back side. Inthis way, the second substrate S2 is completely severed parallel to thegrooves.

The two-stage execution of the separation has the advantage that theincisions can be produced with a substantially more imprecise tool,which would not be usable to carry out the separation simply because ofthe relatively great deviation in the depth of the incision. Theresulting lower limit for the depth of the groove is therefore that thedepth of the groove should at least correspond to the tolerance whichcorresponds to the tool for producing the incisions, in particular thediamond saw DS. If there is sufficient free clearance because of aparticular configuration of the surface topology of the two substrateson the connecting side between the top rim of the grooves in the secondsubstrate and the surface of the first substrate below it, the depth ofthe groove can be reduced by that amount. High quality diamond saws usedtoday have an incision depth tolerance of more than 20 μm, normally50-60 μm. As a result, the depth of the grooves preferably correspondsto the maximum sustainable tolerance of the saw. If both substrates S1,S2 are semiconductor wafers which have for example a thickness of 500μm, then the necessary groove depth for the forenamed high-quality sawsis for example only about 10% of the thickness of the layer. Thisresults in only a trivial degrading of the strength of the wafer. Thegreater mechanical strength of the substrate S according to the presentinvention makes it possible then to use larger wafers for the productionof COC components, or better of stacked wafer components in general,without fear of a greater danger of breakage during the manufacturingand processing procedure, in particular in the case of wafer bonding.The second incision ES is represented in the figure only by dashedlines.

The figure shows directly an additional advantage of the methodaccording to the present invention. The width of the cut when producingthe incisions can be chosen at will, simplifying the adjustment of theincisions relative to the grooves. It is even possible to use a tool forproducing the incisions whose width of cut corresponds to the width ofthe cutout, i.e., to the distance between the grooves. In this way, asingle incision is sufficient for a cutout, in order to open the windowto expose the terminal pads AF. However the forenamed diamond saws DSnormally have a cutting width of only about 30-100 μm, whereas asufficient width of the cutout AS according to present-day wire bondingtechnology should be at least 0.5 mm, but preferably 1 to 3 mm.

After the incisions have been completed, the substrate stripcorresponding to the cutout AS is completely severed from the rest ofthe substrate S2 and can be removed. In the resulting window F theterminal pads AF are now exposed, as shown in schematic cross section inFIG. 5, and because of the appropriate width of the cutout AS they arereadily accessible for a soldering process. In the figure there are twoterminal pads in the window F, which belong to different components areidentified correspondingly by indices A and B. The separating lineTL_(v), along which the two components will later be separated, issketched in with dashed lines.

FIG. 6 shows a top view of the arrangement at this stage of theprocedure. In the selected section of the component, which is largeroverall, there is a crossing of two separating lines TL_(v), TL_(h), sothat the abutting corners of four individual components are shown here.For each individual component there can be any number of terminal padsprovided, depending on the type of component. For each wafer (substrate)the number of individual components is limited essentially by the sizeof the individual components, but is otherwise open.

At this stage it is now already possible to test the individualcomponents for their functionality, and to that end in particular toprovide the terminal pads AF with corresponding contacts. It is alsopossible, however, to separate the components directly. This can be donein two stages, with the arrangement being separated with the help of asuitable separating tool, for example the forenamed diamond saw, alongthe separating lines TL of one sort, for example TL_(v), into individualstrips, with each strip containing a row of joined components. Theelectrical test of the individual components can also be done at thisstage, before they are subsequently separated into individual componentsalong the second separating lines TL_(h).

FIG. 7 shows part of an individual component after separation. It canreadily be seen that the terminal pads AF are adjacent in the edge areaof the particular component or its first substrate. The figure alsoshows now the terminal pads are connected to a bonding wire (BD) withthe help of a metallic connection, for example a bump BP, and makeoutside contact, for example on a circuit board. The positioning of theterminal pads AF on the edge has the advantage that the bonding wire canbe routed from the edges of the second substrate S2 without motion. Ifadditional contacts are provided on the second substrate S2 forcomponent structures within the second substrate, this eliminates thedanger of an unwanted short circuit.

With the help of the method according to the present invention, thesubmerged terminal pads of a great variety of components can be exposed,which were originally covered during production under two substratesthat were joined together over a large area. Accordingly, the componentcan be formed in only one substrate, in particular in the firstsubstrate S1, and the second substrate S2 can therefore be for example acover for the component. In all cases the two substrates are ceramic,glass-like or crystalline, but in any case rigid, firm bodies with largesurfaces. There can also be a vacuum or any desired gaseous atmosphereenclosed in cavities which may be formed between the two substrates S1and S2. At least one of the two substrates can also have micromechanicalor micro-optical component structures. Preferably the component istherefore an IC, a sensor, a MEMS Sensor for pressure, acceleration orspeed of rotation, a micro-optical component such as a photo sensorarray with a glass cover, or any other component that includes twosubstrates joined to each other.

In an additional embodiment of the present invention, not shown, thecomponent has a structure consisting of more than two stackedsubstrates. The third and additional substrates can be placed on thesurface of the first or the second substrate. Preferably, the additionalsubstrates are joined with one of the two other substrates or with thecomposite of the two substrates between the procedural stages shown inFIGS. 2 and 4. A third or additional substrate, lying with its wholesurface on the second substrate S2, can form a unit according to thepresent invention with the second substrate, which unit is treated likea single second substrate and is completely severed when the incision ESis produced.

1. A method of producing a component comprised of at least first and second substrates, the first substrate having an upper surface that contains terminal pads, and the first substrate comprising component structures that are electrically conductive and that are electrically connected to the terminal pads, the second substrate having a lower surface that faces the upper surface of the first substrate, the method comprising: forming grooves having a predefined depth on the lower surface of the second substrate; forming incisions on an upper surface of the second substrate, the incisions reaching the grooves to form a cutout portion in the second substrate; and removing the cutout portion to expose the terminal pads.
 2. The method of claim 1, wherein the grooves and the incisions are formed in substantially straight lines on the second substrate; wherein the incisions are produced by sawing; and wherein the cutout portion is defined between a pair of grooves.
 3. The method of claim 2, wherein the terminal pads are positioned side by side in a row on the upper surface of the first substrate; and wherein the cutout portion exposes the row of terminal pads.
 4. The method of claim 1, wherein the grooves are formed via wet chemical etching, ion beam etching, or plasma etching.
 5. The method of claim 4, wherein the grooves are defined by a resistance mask that is structured photolithographically.
 6. The method of claim 1, wherein the grooves are formed by laser cutting.
 7. The method of claim 2, wherein the grooves are formed to a depth that is greater than a cutting depth precision of a sawing process used to form the incisions.
 8. The method of claim 1, further comprising: joining the substrates; and shaping at least one of the first and second substrates to produce a clearance between the terminal pads and the second substrate.
 9. The method of claim 8, further comprising: before joining the first and second substrates, applying a covering over at least the terminal pads to prevent the second substrate from adhering to the terminal pads; and removing the covering after removing the cutout portion.
 10. The method of claim 8, wherein the first and second substrates are joined via glass bonding, bonding by means of bumps, anodic bonding, eutectic bonding, direct bonding of substrate surfaces, or gluing.
 11. The method of claim 1, wherein the first and second substrates comprise wafers; and wherein the method further comprises separating individual components from the first and second substrates after the cutout portion is removed.
 12. The method of claim 11, wherein terminal pads of individual components are on edges of the individual components; and wherein removal of the cutout portion exposes two adjacent rows of terminal pads for adjacent components.
 13. The method of claim 1, wherein at least one of the first and second substrates comprises microelectrical components, micro-optical components, micro-mechanical components or a combination thereof.
 14. A method of producing a component comprised of first and second substrates that are joined via an upper surface of the first substrate and a lower surface of the second substrate, the upper surface of the first substrate containing terminal pads, and the first substrate comprising component structures that are electrically conductive and that are electrically connected to the terminal pads, the method comprising: forming grooves having a predefined depth on the lower surface of the second substrate, the grooves being formed in substantially straight lines via a first formation technique, a pair of the grooves defining a strip-shaped cutout portion of the second substrate; joining the upper surface of the first substrate and the lower surface of the second substrate; forming incisions on an upper surface of the second substrate via a second formation technique, the incisions reaching the grooves to separate the cutout portion from a remainder of the second substrate, the first formation technique differing from the second formation technique, and the first formation technique having greater precision than the second formation technique; and removing the cutout portion to expose the terminal pads.
 15. The method of claim 14, wherein the grooves are formed via wet chemical etching, ion beam etching, or plasma etching.
 16. The method of claim 14, wherein the grooves are defined by a resistance mask that is structured photolithographically.
 17. The method of claim 14, wherein the grooves are formed by laser cutting.
 18. The method of claim 14, further comprising: shaping at least one of the first and second substrates to produce a clearance between the terminal pads and the second substrate.
 19. The method of claim 14, further comprising: before joining the first and second substrates, applying a covering over at least the terminal pads to prevent the second substrate from adhering to the terminal pads; and removing the covering after removing the cutout portion.
 20. The method of claim 14, wherein the first and second substrates are joined via glass bonding, bonding by means of bumps, anodic bonding, eutectic bonding, direct bonding of substrate surfaces, or gluing. 